1. Field of the Invention
This invention relates to a grain-oriented electromagnetic steel sheet used as a core material of transformers and power generators, especially to a grain-oriented electromagnetic steel sheet having low iron loss and excellent strain resistance and excellent performance in use.
2. Description of the Related Art
Grain-oriented electromagnetic steel sheets containing Si having crystal grains oriented along the (110) {001} or (100) {001} direction are widely used for various kinds of iron cores operated at commercial frequencies because of good soft-magnetic properties. An important property required of this kind of electromagnetic steel sheet is low iron loss (generally represented by electric loss W17/50 (W/kg) when the steel sheet is magnetized to 1.7 T at a frequency of 50 Hz).
Methods for reducing the iron loss of a steel sheet include increasing electric resistance by adding Si which is effective for reducing eddy current loss of a steel sheet, or reducing the thickness of the steel sheet, or making the grain diameter small, or aligning the orientation of grains that are effective for reducing hysteresis loss.
Among those methods, addition of Si encounters limitations since decrease of saturation magnetic flux density may be induced when the amount of Si is excessive, and expansion of iron core size is caused. Reducing the thickness of the steel sheet, on the other hand, tends to result in excessive production cost increase.
Accordingly, recent technical developments for reducing iron loss have concentrated on improving alignment of crystal orientations and reducing the grain size in the steel. The alignment of orientations can usually be evaluated by magnetic flux density B8 (T) at a magnetization strength of 800 A/m. However, the alignment of orientations should be optimized, i.e., the B8 value should be adjusted to its optimum in order to obtain minimum iron loss, because an inconsistent relationship exists wherein improving the alignment of crystal orientations inevitably results in an increase of grain diameter and hence deterioration of iron loss.
The requirement to make the grain diameter small for reducing the iron loss has been eliminated thanks to the recent technical development by which the width of magnetic domains can be finely divided artificially by irradiating with a plasma jet or laser beam. Therefore, the method for reducing the iron loss by increasing the alignment of orientations has became a leading technique today, allowing development of a material having a magnetic flux density (B8) of as large as 1.93 to 2.00 T.
Processing methods developed for finely dividing magnetic domains include not only forming linear grooves or introducing linear local stress, but also smoothing the roughness of the interface between the surface of the steel sheet and the non-metallic coating film, or applying crystal orientation emphasis on the surface of the metal. Finely dividing the magnetic domains enabled some improvement of iron loss characteristics.
It is necessary that secondary recrystallization is perfectly controlled to enhance the alignment of orientations. In secondary recrystallization growth of normal crystal grains can be suppressed by finely dispersing precipitates of inhibitors such as AlN, MnSe or MnS, thereby allowing growth of large grains along a specified preferable ((110) [001]) direction and nearby directions referred to as Goss directions. Inhibitor elements tending to segregate at grain boundaries, such as Sb, Sn and Bi, are also used as sub-inhibitors.
Production of electromagnetic steel sheets having a high magnetic flux density as described above has involved combining the foregoing techniques with a technique adapted to control the aggregated textures of crystal grains.
When a transformer was produced using a grain-oriented electromagnetic steel sheet having good soft-magnetic properties, however, the transformer often failed to have the characteristics required for practical use. This is especially true in the case of a laminated transformer where the steel sheet is used without applying stress-relief annealing after shear processing, which causes discrepancies between the characteristics of the materials and especially the performance a large transformer. Performance in final usage is referred to herein generically as xe2x80x9cperformance of a practical device.xe2x80x9d
There have been problems in the prior art that expected characteristics suitable for practical devices cannot always be obtained even when a transformer is produced by using a grain-oriented electromagnetic steel sheet having a high magnetic flux density. This is an intrinsic problem when a material having a high magnetic flux density is used. It was elucidated that an undesirable distorted flow of the magnetic flux that causes digression of the magnetic flux from its flow direction takes place at the T-shaped junction of the transformer, so that reduction of the iron loss cannot be attained. This problem was considered to be beyond improvement.
However, the practical performance of a transformer or other device is largely deteriorated even when recent materials are used in which the flux density has been much more improved.
The phenomenon, wherein iron loss characteristics deteriorate under shear processing and lamination, was observed as being accompanied by improvement of magnetic flux density. This phenomenon is still under investigation. The only countermeasures now available at hand are to suppress addition of strain as much as possible, by careful handling of the material.
Although it is doubtless true that iron loss characteristics have been improved by various techniques for finely dividing magnetic domains as described above, yet there remain problems, since the desired characteristics cannot be attained when a practical device is produced using the materials now available, especially when the device is used in a high magnetic field.
The method step of imparting high magnetic flux density to the grain-oriented steel sheet has been known in the art and elements such as Al, Sb, Sn and Bi are effective for the purpose.
A value of 1.981 T is reported in Japanese Examined Patent Publication No. 46-23820 as B10 (the magnetic flux density under a magnetic field strength of 1000 A/m) in a grain-oriented electromagnetic steel sheet containing Al and S, while a value of 1.95 T is reported in Japanese Examined Patent Publication No. 62-56923 as B8 in a grain-oriented electromagnetic steel sheet containing Al, Se, Sb and Bi as inhibitors.
The magnetic properties of these grain-oriented electromagnetic steel sheets are splendid, but when a transformer is produced using these electromagnetic steel sheets having a desired value for iron loss of the resulting device cannot be often obtained. This is believed to originate, as hitherto described, from a high alignment of crystals that cannot be avoided.
Accordingly, an object of the present invention is to provide a grain-oriented electromagnetic steel sheet without causing deterioration of performance while improving the magnetic characteristics of the material. We have accordingly studied the reasons, in a material having secondary recrystallized grains that are highly aligned, why the performance is largely deteriorated below the level presumed because of iron loss of the material, and why the material is so sensitive to strain applied during further processing steps. As a result, we have discovered the following procedures.
We have investigated a variety of causes affecting distorted flow of the magnetic flux at the T-shaped junction parts of laminated transformers in which a material of high magnetic flux density is used.
It was found for the first time that the cause of deterioration is not only a highly aligned orientation but also by the grain diameter.
Meanwhile, the following facts were also found with respect to the effect of strain introduced during further processing of the sheet.
Iron loss is reduced due to refinement of magnetic domains. Generally, magnetic domains are divided by the mechanism that finely divided domains can reduce magnetostatic energy once increased by the appearance of magnetic poles at grain boundaries or on surfaces of steel sheets. Therefore, the generation of magnetic poles is the origin of reducing iron loss.
In materials having a high alignment of grain orientations, more magnetic poles appear at the grain boundaries than on the surface of the steel sheet. Moreover, the distances between the grain boundaries become large because of large grain diameters in these materials, which makes magnetostatic energy generate weakly. The introduced strains suppress the generation of magnetic poles more strongly inside the steel than on the surface. Thereby, in these materials, the increment of magnetostatic energy caused by magnetic poles at grain boundaries or by those in domain refinement area is reduced by disappearing magnetic poles through introducing strains, resulting in the enlargement of magnetic domain and in increase in iron loss.
While, in the cause of the materials having small grains and a low alignment of grain orientations, magnetic poles appear preferably on the surface of the steel, which makes iron loss of these material stable against introducing strains. We have discovered that this is the reason why an electromagnetic steel sheet with high magnetic flux density is so sensitive to strain.